The present invention is related to a solid electrolytic capacitor comprising a pressed powder anode and an anode wire extending therefrom. More specifically, the present invention is related to a process for improving the connection between the anode wire and anode thereby allowing for the use of powders with a higher charge density per gram (CV/g).
There is an ever increasing demand for electronics with higher capability, lower cost, and better durability. This demand, at the electronics level, places a burden on the component manufacturers to provide more functionality and, often, this increased functionality is required in the same, or less, volume of the electronics. With regards to capacitors this has led to the constant desire to provide higher volumetric efficiency or capacitance per volume of capacitor.
Solid electrolytic capacitors comprising a pressed powder anode with a wire extending therefrom, for connectivity, are well known in the art. The anode wire is in electrical contact with the anode. The anode wire is typically attached to the anode in one of two ways. In one method common to the art the wire is inserted into the anode powder and secured in the powder during the pressing operation. In a second method the anode wire is welded to the anode after pressing. In each case the anode is sintered and, in the case of a welded wire, prior to anode wire attachment.
The ever constant desire for increased volumetric efficiency has lead to the development of anode powders with increasing CV/g. Unfortunately, these new powders have proven to be very difficult to utilize since the particles necessary to achieve the high CV/g fail to form an adequate bond to the anode wire during the pressing and sintering steps. As the CV/g increases, and the anode size decreases, the problem of inadequate bonding to the anode wire is exasperated. This is a particular problem when deoxygenation sintering is utilized as described in commonly assigned U.S. Pat. No. 8,349,030 to Hussey et al. which is incorporated herein by reference. The deoxygenation further erodes the bond between the anode and anode wire often leading to complete dissociation of the two.
One option is to sinter the anode to a higher degree thereby improving the bond strength between the anode wire and anode. The increased sintering temperatures are believed to form more adequate necks between the particles of the powder to gain sufficient wire to anode mechanical strength. Unfortunately, increased sintering reduces the surface area of the anode metal in the anode and thus the CV/g, and ultimately decreasing the capacitance of the resulting capacitor, which eliminates the advantages offered by the more expensive higher charge powder.
Welding the wire to the formed anode is one approach to addressing this problem. In the welding process a broad welding nugget forms inside and outside of the anode edge. One problem with the welded wire is the requirement for double sintering. The first sintering is before the welding to provide mechanical strength to the anode and the second sintering is after the welding to clean the welding nugget. In addition, the use of welded wires on high charge powders and double sintering is problematic due to the oversaturation of the powder particles with oxygen and precipitation of the crystalline oxide.
There is currently no suitable way to utilize the high charge powders to their fullest extent due to the ineffective adhesion between the anode powder and anode wire. Provided herein is an improved method of forming the connection between the anode and anode wire resulting in an improved capacitor.